Undesirable markings are formed in a fabric when a faulty weft thread has been bound into the fabric and beat-up and then must be removed again. Such markings reduce the quality of the fabric and must be avoided. The removal of the faulty weft thread requires the reverse weaving, for example, if a broken weft thread has already been beat-up against the beat-up edge of the fabric by the respective motion of the reed. As soon as a sensor recognizes the presence of a broken weft thread, the loom is stopped. However, due to inertia the stopping cannot be so instantaneous that beat-up of the broken weft thread is avoided. Rather, the broken weft thread is still bound-in and beat-up because the motor driven main shaft of the loom, the reed drive shaft, and the heald drive shaft of the loom come to a standstill only after about 60.degree. of rotation following the reed beat-up. The heald drive shaft of the heald loom section also participates in this additional rotation because it is connected through a power transmission with the main shaft of the loom. When standstill has been achieved, the loom shed is opened again by reverse rotation of the involved loom components through a number of angular degrees in the reverse direction sufficient to open the loom shed. Preferably, the opening of the loom shed is performed simultaneously with a controlled reverse rotation of the warp beam and of the cloth or fabric take up roller. When the loom shed is sufficiently opened, the faulty or broken weft thread is removed and the loom main shaft as well as the reed shaft are rotated in the forward direction into an angular rotational position that is required for restarting the loom.
A stopping of the weaving operation and thus of the loom drive may be necessary due to the most varied causes. For example, the weaving operation must be interrupted when a faulty weft thread has been inserted into the loom shed and the switching of the upper and lower warp threads has bound the faulty weft thread into the fabric, whereby the stopping is initiated after the detection of the presence of a faulty weft thread. As mentioned, in such an instance the main drive shaft of the loom stops at a rotational angle of about 60.degree. following the beat-up of the weft thread by the reed. The defective weft thread is thus bound-in and beat-up in the finished part of the fabric web being woven.
In order to eliminate the so-called weft fault, it is unavoidable that the loom shed must be opened again by the above mentioned reverse rotation of about 60 or more rotational degrees of the main loom shaft and by also reverse rotating the heald drive shaft. Once the loom shed is sufficiently open, the defective beat-up weft thread is removed from the fabric portion. Such removal may take place in any known manner.
After the defective weft thread has been removed it is necessary to return all the loom components to a position in which these components have been prior to the beat-up of the defective weft thread. These components include not only components rotated by the main shaft, but also the electronic controls and mechanical elements of the heald section. In looms wherein the loom main shaft serves simultaneously as a drive for the heald section, the loom and the heald loom section are rigidly coupled through gear drive means. When in such looms the coupling is disconnected, the reed necessarily performs a further beat-up motion prior to returning of the reed into its back position. Such additional beat-up motion of the reed is unavoidable in the type of loom just mentioned. This additional beat-up leads to undesirable markings or so-called weft stripes that become frequently visible only after further treatment of the fabric such as dyeing or mercerizing. The markings occur because the forward beat-up motion compresses the fabric along the beat-up line and the fabric tries to expand along the beat-up line when the reed moves back.
German Patent Publication (DE-OS) 4,137,681 discloses an effort to avoid these fabric markings by shifting the fabric, or rather the fabric beat-up line, in the direction of the fabric withdrawal from the loom. The just mentioned German Patent Publication discloses the operation of devices that are intended to protect the fabric beat-up edge against the beat-up by the reed during the so-called reverse weaving. For this purpose these devices become operational during reverse weaving that is during the slow reverse rotation of the main loom shaft or of the loom motor that is connected with the main shaft. These devices shift the fabric edge and thus the beat-up edge or line prior to a renewed beat-up by the reed, out of the normal position by a certain distance in the direction of the fabric withdrawal. Once the reverse weaving is completed, these devices return the fabric and thus the fabric beat-up line or edge into the normal position. It is a disadvantage of the just described device according to German Patent Publication (DE-OS) 4,137,681 that a substantial control effort and expense is necessary for the mentioned fabric shifting.
It is also known to decouple the heald loom section from the driving side of the loom main drive shaft after the loom had been stopped in response to the detection of a weft fault. In such a case, a separate drive that is operatively coupled to the drive shaft of the heald loom section takes care of the reverse rotation of the loom main drive shaft, whereby, through the heddles the previous beat-up is opened up again and the warp threads release the defective weft thread without any movement of the reed. In other words, in this type of operation, the loom shed is reproduced in its state when the defective weft thread has been introduced. After removal of the defective weft thread the loom is started again and the preceding weft thread insertion is repeated without any beat-up motion of the reed prior to such repeated weft thread insertion.
German Patent Publication (DE-OS) 2,509,665 discloses a loom with a shed forming mechanism particularly for a heald loom or a Jacquard loom in which the shed forming mechanism and if applicable, a weft thread selection mechanism, are connectable through a main shaft clutch, either to the main shaft or to a reverse drive device. Thus, when a weaving fault, such as a weft thread fault occurs, the shed forming mechanism and the weft thread selection device, if such device is used, are decoupled from the main drive shaft and coupled to the reverse drive mechanism, whereby the latter comprises a reverse drive motor and a reverse switching clutch. In such a conventional loom, the reverse drive device is so constructed that the removal of a weaving fault can be accomplished in substantially less time than without such a device. For this purpose the just mentioned German Patent Publication suggests that a stepping gear drive is arranged between the reverse drive clutch and the reverse drive motor. The stepping gear drive is so constructed that it has a rather increased reduction ratio between the initial step and the last step of the stepping. However, German Patent Publication (DE-OS) 2,509,665 does not mention that the backward weaving normally entails a renewed beat-up of the reed against the fabric edge which leads to a densification of the fabric and to the so-called stripe formation. Besides, the just mentioned known loom requires a separate reverse drive motor for the reverse weaving operation performed by the reverse drive mechanism.
German Patent Publication (DE-OS) 2,706,646 describes a solution to the above problems in looms in such a way that in response to a weaving fault the operational motion of the reed is stopped in a preliminary stopping position. Once the reed is stopped, it is brought into a desired stopping position by a positioning motion which is slower than the normal operational motion. This known solution to the above problem further uses the loom drive motor also for the slow drive of the reed or loom sley. For this purpose the loom drive motor is an electric motor having a variable r.p.m. However, German Patent Publication (DE-OS) 2,706,646 also mentions that a separate motor may be provided for this slow drive in addition to the regular loom drive motor. This additional motor also serves for the backward motion of the heald loom section that is for the backward weaving. Thus, at least in connection with looms equipped with a heald loom section it is necessary to provide, in addition to the normal gear drives, a coupling mechanism for decoupling and again coupling the loom drive from or to the heald loom section thus requiring at least one additional reverse running motor.